The present invention relates to a lithium secondary battery used as a power source for memory retention in electronic equipment, a power source for driving portable electronic equipment, or the like, to an anode for a lithium secondary battery, and to a method for manufacturing the anode.
In recent years, lithium batteries which have lithium salts as electrolytic components have attracted attention since such batteries give a high energy density at a high voltage (3 to 4V), and such batteries have been developed to the extent that they can be used in practical applications. In order to make personal computers, word processors, portable telephones and the like yet more portable, it will be necessary in the future to make lithium batteries into the form of rechargeable secondary batteries and to further improve cycle characteristics (charging efficiency, cycle life, etc.).
A lithium secondary battery has a cathode capable of charging and discharging lithium ions, an anode comprising a material which may be doped and de-doped with lithium ions, lithium metal or the like, and an electrolyte that allows migration of lithium ions. A nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent is generally used as the electrolyte.
However, with lithium metal batteries that use metallic lithium or a lithium alloy as an anode active substance, the electrolytic solution has a tendency to decompose due to reaction between the organic solvent in the electrolytic solution and the anode active substance. There is thus a problem that good cycle characteristics cannot be obtained with lithium metal batteries that use an electrolytic solution containing an organic solvent.
The invention disclosed in JP-A-10-261435 attempts to solve this problem. The invention in this document attempts to suppress the reaction between the organic solvent and the anode active substance by adding an imide compound to the electrolytic solution. This method certainly gives good results with xe2x80x98coin-shapedxe2x80x99 batteries. However, with cylindrical batteries it is difficult to put in a large quantity of the electrolytic solution, meaning that it is difficult to put in a large quantity of the imide compound. With the method disclosed in the above-mentioned publication, it is thus difficult to sufficiently suppress the reaction between the organic solvent and the anode active substance in the case of a cylindrical lithium metal battery.
One of the problems with lithium secondary batteries is thus that good cycle characteristics cannot be obtained for a variety of different battery forms.
An object of the present invention is to solve this problem.
The present invention provides, as a first aspect, a lithium secondary battery that comprises a cathode capable of charging and discharging lithium ions, an anode containing a material which can be doped and de-doped with lithium ions, lithium metal, or a lithium alloy, and an electrolyte that allows migration of lithium ions, wherein the aforementioned anode also contains an imide compound.
Imide compounds have a strong ability to coordinate with metal ions. It is thought that in an electrolyte containing an organic solvent and lithium ions, the imide compound coordinates with the lithium ions more quickly and more strongly than the organic solvent coordinates with the lithium ions, so that the imide compound exists in the form of coordination complex with the lithium ions. It can thus be envisaged that, even if an electrolyte containing an organic solvent is used, reaction between the organic solvent and the anode (the anode active substance) is suppressed due to the presence of the imide compound(s) in the lithium secondary battery of the present invention. Consequently, reaction between the organic solvent and the anode active substance can be adequately avoided and the cycle characteristics of the lithium secondary battery improved, regardless of whether the lithium secondary battery has a coin-shaped form or a cylindrical form.
Lithium secondary batteries can broadly be categorized into lithium metal secondary batteries and lithium ion secondary batteries. The technical ideas of the present invention can be applied to either type of lithium secondary battery.
In the lithium metal secondary battery of the present invention, the cathode is, for example, composed of a mixture comprising a cathode active substance that is capable of occluding and releasing lithium ions, a conductant agent that has the function of supplementing the conductivity of the cathode, and abinder for binding the cathode active substance and the conductant agent together.
The above-mentioned cathode active substance is, for example, a macromolecular conductive material, a metal oxide, a metal sulfide, an inorganic conductive material, or the like.
Examples of the above-mentioned macromolecular conductive material include polyaniline, polyacetylene, poly-p-phenylene, polybenzene, polypyridine, polythiophene, polyfuran, polypyrrole, polyanthracene and polynaphthalene, along with derivatives of these macromolecular compounds.
Examples of the above-mentioned metal oxide include manganese dioxide, vanadium pentoxide, molybdenum trioxide, chromium trioxide and cupric oxide.
Examples of the above-mentioned metal sulfide include molybdenum disulfide, titanium disulfide and iron disulfide.
Examples of the above-mentioned inorganic conductive material include fluorocarbons.
The above-mentioned conductant agent is, for example, acetylene black, graphite, carbon, or the like.
The above-mentioned binder is, for example, Teflon resin, an ethylene-propylene-diene terpolymer, or the like.
The anode used in the lithium metal secondary battery of the present invention comprises, for example, an anode active substance and an imide compound.
The above-mentioned anode active substance is, for example, metallic lithium or a lithium alloy. In the anode, the anode active substance is in the form, for example, of foil or a plate.
An example of the above-mentioned lithium alloy is an alloy of metallic lithium and at least one metal selected from the group consisting of metals such as aluminum, magnesium, indium, mercury, zinc, cadmium, lead, bismuth, tin and antimony.
Specific examples of the above-mentioned lithium alloy include a lithium-aluminum alloy, a lithium-tin alloy and a lithium-lead alloy.
Examples of the above-mentioned imide compound are compounds represented by undermentioned general formula (1). 
Here, Z is an optionally substituted xe2x80x94(CH2)nxe2x80x94 (where n is an integer from 2 to 7), 1,2-cyclohexylene, or 1,2-phenylene. X is a hydrogen atom, or an optionally substituted alkyl group, aralkylcarbonyl group, alkylcarbonyl group, alkoxycarbonyl group, aralkyloxycarbonyl group, or imidyloxycarbonyl group.
xe2x80x98xe2x80x94(CH2)nxe2x80x94 (where n is an integer from 2 to 7)xe2x80x99 in the definition of Z includes alkylene radicals such as ethylene, trimethylene, tetramethylene, pentamethylene, hexamethylene and heptamethylene. n is preferably 2 or 3.
Possible substituents in the definition of Z include lower alkyl groups, lower alkoxy group, and residues that can form a cyclic imide with the atoms that make up Z.
In view of the above, Z is preferably ethylene, trimethylene, 1,2-cyclohexylene, 1,2-cyclopentylene or 1,2-phenylene, and out of these most preferably ethylene or 1,2-phenylene.
The alkyl group in the definition of X may be either a straight chain alkyl group or a branched alkyl group, and preferably contains 1 to 20 carbon atoms, more preferably 1 to 6 carbon atoms.
Examples of the above-mentioned straight chain alkyl group include methyl, ethyl, propyl, butyl, pentyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl and icosyl.
Examples of the above-mentioned branched alkyl group include isopropyl, methylpropyl, methylbutyl, methylpentyl, methylheptyl, methyloctyl, methylnonyl, methyldecyl, methylundecyl, methyldodecyl, methyltridecyl, methyltetradecyl, methylpentadecyl, methylhexadecyl, methylheptadecyl, methyloctadecyl and methylnonadecyl.
An example of the aralkylcarbonyl group in the definition of X is an alkylcarbonyl group containing 1 to 6 carbon atoms that is substituted with an aryl group such as phenyl or naphthyl.
Specific examples of the above-mentioned aralkylcarbonyl group include benzylcarbonyl, phenethylcarbonyl, phenylpropylcarbonyl, phenylbutylcarbonyl, phenylpentylcarbonyl and phenylhexylcarbonyl.
The alkylcarbonyl group in the definition of X may be either a straight chain alkylcarbonyl group or a branched alkylcarbonyl group, and preferably contains 1 to 20 carbon atoms, more preferably 1 to 6 carbon atoms.
Examples of the above-mentioned straight chain alkylcarbonyl group include acetyl, ethanoyl, propanoyl, butanoyl, pentanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, undecanoyl, dodecanoyl, tridecanoyl, tetradecanoyl, pentadecanoyl, hexadecanoyl, heptadecanoyl, octadecanoyl, nonadecanoyl and icosanoyl.
Examples of the above-mentioned branched alkylcarbonyl group include isopropanoyl, methylpropanoyl, methylbutanoyl, methylpentanoyl, methylheptanoyl, methyloctanoyl, methylnonanoyl, methyldecanoyl, methylundecanoyl, methyldodecanoyl, methyltridecanoyl, methyltetradecanoyl, methylpentadecanoyl, methylhexadecanoyl, methylheptadecanoyl, methyloctadecanoyl and methylnonadecanoyl.
The alkoxycarbonyl group in the definition of X may be either a straight chain alkoxycarbonyl group or a branched alkoxycarbonyl group, and preferably contains 1 to 20 carbon atoms, more preferably 1 to 6 carbon atoms.
Examples of the above-mentioned straight chain alkoxycarbonyl group include methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, heptoxycarbonyl, octoxycarbonyl, nonoxycarbonyl, decoxycarbonyl, undecoxycarbonyl, dodecoxycarbonyl, tridecoxycarbonyl, tetradecoxycarbonyl, pentadecoxycarbonyl, hexadecoxycarbonyl, heptadecoxycarbonyl, octadecoxycarbonyl, nonadecoxycarbonyl and icosoxycarbonyl.
Examples of the above-mentioned branched alkoxycarbonyl group include isopropoxycarbonyl, methylpropoxycarbonyl, methylbutoxycarbonyl, methylpentoxycarbonyl, methylheptoxycarbonyl, methyloctoxycarbonyl, methylnonoxycarbonyl, methyldecoxycarbonyl, methylundecoxycarbonyl, methyldodecoxycarbonyl, methyltridecoxycarbonyl, methyltetradecoxycarbonyl, methylpentadecoxycarbonyl, methylhexadecoxycarbonyl, methylheptadecoxycarbonyl, methyloctadecoxycarbonyl and methylnonadecoxycarbonyl.
An example of the aralkyloxycarbonyl group in the definition of X is an alkoxycarbonyl group containing 1 to 6 carbon atoms that is substituted with an aryl group such as phenyl or naphthyl.
Specific examples of the above-mentioned aralkyloxycarbonyl group include benzyloxycarbonyl, phenethyloxycarbonyl, phenylpropyloxycarbonyl phenylbutyloxycarbonyl, phenylpentyloxycarbonyl and phenylhexyloxycarbonyl.
Examples of the imidyloxycarbonyl group in the definition of X include succinimidyloxycarbonyl and phthalimidyloxycarbonyl.
Out of the examples given for X, particularly preferable ones are a hydrogen atom, alkoxycarbonyl groups containing 1 to 6 carbon atoms, and aralkyloxycarbonyl groups for which the alkyl part contains 1 to 6 carbon atoms.
Examples of the substituents in the definition of X include lower alkyl groups, lower alkoxy groups, and a succinimidoxycarbonyloxy group.
In summary, imide compound(s) of the above-mentioned chemical formula (1) are preferably used, in which Z is ethylene(succinimide type compound) or 1,2-phenylene (phthalimide type compound), and X is a hydrogen atom, an alkoxycarbonyl group containing 1 to 6 carbon atoms, or an aralkyloxycarbonyl group for which the alkyl part contains 1 to 6 carbon atoms.
More specifically, a preferable choice for one of the above-mentioned imide compound(s) is a compound represented by undermentioned chemical formula (2) or (3). 
In above-mentioned chemical formula (2), X1 is optionally substituted xe2x80x94(CH2)nxe2x80x94 (where n is an integer from 2 to 20), 1,2-cyclohexylene, or 1,2-phenylene. 
In above-mentioned chemical formula (3), each X is the same as X in above-mentioned chemical formula (1), and Z2 is phenyl tetrayl, binaphthyl tetrayl, isopropylidene diphenyl tetrayl, hexafluoroisopropylidene diphenyl tetrayl, diphenyl ether tetrayl, diphenyl ketone tetrayl or diphenyl sulfone tetrayl.
Specific examples of the above-mentioned imide compound(s) include those represented by undermentioned chemical formulae (A) to (G). Specifically, N-hydroxyphthalimide (undermentioned chemical formula (A)), N-hydroxysuccinimide (undermentioned chemical formula (B)) N,N-disuccinimidyl carbonate (undermentioned chemical formulae (C)), 1,5-bis(succinimidoxycarbonyloxy)pentane (undermentioned chemical formula (D)), 9-fluorenylmethyl-N-succinimidyl carbonate (undermentioned chemical formula (E)), N-(benzyloxycarbonyloxy)succinimide (undermentioned chemical formula (F)), and Z-glycine-N-succinimidyl ester (undermentioned chemical formula (G)), are preferable choices for the above-mentioned imide compound(s). 
The imide compound content in the anode is made to be in the range 1 to 30 wt % relative to the total weight of the anode. This is because, if the imide compound content is too low, then it will not be possible to sufficiently improve the cycle characteristics (charging/discharging efficiency, cycle life, etc.) of the lithium secondary battery, whereas if the imide compound content is too high, then the proportion of the anode active substance in the anode will be unreasonably low. In order to obtain even better results, the imide compound content is made to be in the range 5 to 20 wt %.
When the electrolyte used is in the form of an electrolytic solution, the one or more electrolytic components are dissolved in an organic solvent.
The electrolytic component used in the above-mentioned electrolyte can be any of those commonly used in the technical field in question, for example lithium salt. The lithium salt used as the electrolytic component in the lithium secondary battery of the present invention may be either an inorganic salt or organic salt.
Examples of the above-mentioned inorganic salt include LiPF6, LiClO4, LiAsF6, LiAlCl4, LiBF4, LiCl and LiBr.
Examples of the above-mentioned organic salt include CH3SO3Li, CF3SO3Li, LiB(C6H5)4 and CF3COOLi.
The lithium salts given as examples here may either be used alone or a plurality may be used in combination.
The organic solvent used can be a publicly-known solvent (for example a high-permittivity solvent or a low-viscosity solvent) used in the technical field in question.
An example of the above-mentioned high-permittivity solvent is a cyclic carbonate having 3 to 5 carbon atoms. Examples of such a cyclic carbonate having 3 to 5 carbon atoms include ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC).
Examples of the above-mentioned low-viscosity solvent include a linear carbonate having 3 to 9 carbon atoms, a linear ether, an ester, and an aromatic hydrocarbon.
Examples of the above-mentioned linear carbonate having 3 to 9 carbon atoms include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC) and methyl ethyl carbonate (MEC).
Examples of the above-mentioned linear ether include 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE) and 1,2-dibutoxyethane (DBE).
Examples of the above-mentioned ester include cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2-MeTHF), methyl formate, methyl acetate, and methyl propenoate.
Examples of the above-mentioned aromatic hydrocarbon include benzene (Bz), toluene and xylene.
The above-mentioned high-permittivity solvents and low-viscosity solvents may either be used alone or a plurality may be used in combination. Note, however, that if low-viscosity solvent is used as the above-mentioned organic solvent, then it is preferable to use this low-viscosity solvent in combination with high-permittivity solvent in order to compensate for the low charging/discharging efficiency of the low-viscosity solvent.
Possible combinations of the above-mentioned high-permittivity solvents and low-viscosity solvents include 2-component solvent systems such as EC-DMC, EC-DEC, PC-DMC, PC-DEC and PC-MEC, 3-component solvent systems such as EC-DMC-Bz, EC-DEC-Bz, PC-DMC-Bz, PC-DEC-Bz, EC-PC-DMC and EC-PC-DEC, and 4-component solvent systems such as EC-PC-DMC-Bz and EC-PC-DEC-Dz.
The ratio of high-permittivity solvent to low-viscosity solvent (by volume) is made to be, for example, 1:4 to 2:1, preferably 1:2 to 1:1.
The above-mentioned electrolyte may also be in the form of a solid electrolyte. Examples of such a solid electrolyte include polyacrylonitrile, poly vinylidene fluoride, photocured polymerizable monomers consisting of ethoxydiethyl glycol acrylate and trimethylolpropane triacrylate, and polyphosphazenes.
On the other hand, when the lithium secondary battery is made to be in the form of a lithium ion secondary battery, the cathode is, for example, composed of a mixture of a cathode active substance, conductant agent and binder.
Examples of the above-mentioned cathode active substance include lithium mixed metal oxides represented by the general formula Lip(MO2)q (where M is at least one metal selected from cobalt, nickel and manganese; p and q are integers satisfying the valencies), and intercalation compounds containing lithium such as LiCoO2, LiNiO2, LiMn2O4 and LiMn3O6.
The above-mentioned conductant agent and binder can be just like the conductant agent and binder used in the cathode of the lithium metal secondary battery.
The above-mentioned anode is, for example, composed of a mixture comprising an anode active substance, an imide compound, a conductant agent and a binder.
A carbon material is preferably used as the above-mentioned anode active substance. Examples of such a carbon material include graphite, a conjugated resin, a fused polycyclic hydrocarbon, and a furan resin.
Examples of the above-mentioned conjugated resin include phenolic resins, acrylic resins, polyimide resins and polyamide resins.
Examples of the above-mentioned fused polycyclic hydrocarbon include naphthalene, phenanthrene and anthracene.
Examples of the above-mentioned furan resin include a homopolymer of furfuryl alcohol, a homopolymer of furfural, and a copolymer of furfuryl alcohol and furfural.
A material obtained by baking/carbonizing an organic material such as oxygen-bridged petroleum pitch can also be used as a carbon material.
The carbon materials given as examples here may either be used alone or a plurality may be used in mixture. Use of graphite is particularly preferable.
The imide compound used in the anode of the above-mentioned lithium ion secondary battery can be just like the imide compound used in the anode of the above-mentioned lithium metal secondary battery. Furthermore, the conductant agent and binder used in the anode of the above-mentioned lithium ion secondary battery can be just like the conductant agent and binder used in the cathode of the above-mentioned lithium metal secondary battery.
Moreover, in both the above-mentioned lithium metal secondary battery and the above-mentioned lithium ion secondary battery, a separator or separators may be provided between the cathode and the anode in order to retain the electrolyte and prevent short-circuiting between the cathode and the anode. There are no particular limitations on the separator material provided it is an easily processable insulating material that is not dissolved by the electrolyte. Specific examples of the separator material include porous polypropylene and porous polyethylene.
In both the above-mentioned lithium metal secondary battery and the above-mentioned lithium ion secondary battery, the cathode and/or the anode may be formed on a collector comprising a metal such as aluminum or copper.
The lithium secondary battery of the present invention can be cylindrical, rectangular, coin-shaped (button-shaped) or sheet-shaped.
In a second aspect of the present invention, an anode used in a lithium secondary battery is provided. The lithium secondary battery anode provided in the second aspect of the present invention is characterized by comprising an anode active substance and at least one kind of imide compound.
Examples of the above-mentioned anode active substance are as described in the above-mentioned first aspect of the present invention. Specifically, examples of the above-mentioned anode active substance include lithium and alloys thereof, and materials which can be doped and de-doped with lithium ions is possible (for example carbon materials).
As the above-mentioned imide compound, those given as examples of the imide compounds included in the anode in the above-mentioned first aspect of the present invention are preferably used. In particular, it is preferable to use at least one selected from the group consisting of N-hydroxyphthalimide, N-hydroxysuccinimide, N,N-disuccinimidyl carbonate, 1,5-bis(succinimidoxycarbonyloxy)pentane, 9-fluorenylmethyl-N-succinimidyl carbonate, N-(benzyloxycarbonyloxy)succinimide, and Z-glycine-N-succinimidyl ester.
The imide compound content in the anode is made to be in the range 1 to 30 wt % relative to the total weight of the anode, more preferably in the range 5 to 20 wt %.
In a third aspect of the present invention, a manufacturing method for the lithium secondary battery anode described in the above-mentioned second aspect of the present invention is provided.
A first method of the above-mentioned manufacturing method is a manufacturing method for a lithium secondary battery anode that comprises an anode active substance and an imide compound, wherein this first method is characterized in that an organic solvent in which the above-mentioned imide compound has been dissolved is applied onto the above-mentioned anode active substance, and then this organic solvent is evaporated.
With this first method, an anode having a multilayer structure consisting of anode active substance layers and imide compound layers is easily provided. The anode having a multilayer structure is formed, for example, by applying the imide compound onto a piece of anode active substance that is in the form of foil or a plate, and then further placing another piece of anode active substance that is in the form of foil or the like on top of the imide compound layer. When forming this anode, further imide compound layers and anode active substance layers may of course be built up, and moreover the resulting layered structure may be rolled to a desired thickness.
The organic solvent used in the above-mentioned first method can be just like the organic solvent used when making the electrolyte into the form of an electrolytic solution in the lithium secondary battery described in the first aspect of the present invention. Out of the examples given for this organic solvent, it is preferable to use dimethyl carbonate (DMC) as the organic solvent in the above-mentioned first method.
A second method of the above-mentioned manufacturing method is a manufacturing method for a lithium secondary battery anode that comprises an anode active substance and an imide compound, wherein this second method is characterized in that the anode active substance is heated together with the imide compound and melted, thus dispersing the imide compound in the anode active substance.
A third method of the above-mentioned manufacturing method is a manufacturing method for a lithium secondary battery anode that comprises an anode active substance and an imide compound, wherein this third method is characterized in that the above-mentioned anode is formed by extrusion molding an ingot comprising the anode active substance, and when doing this applying the imide compound onto the surface of the ingot.
The imide compound may be applied onto the ingot by dissolving the imide compound in an organic solvent and then spraying the resulting solution, or by directly applying the imide compound in powder form. The organic solvent used in the third method can be just like the organic solvent used in the above-mentioned first method.